1- Open file “header.php” in your theme

2- Locate the code:
<img src=”<?php header_image(); ?>” width=”<?php echo HEADER_IMAGE_WIDTH; ?>” height=”<?php echo HEADER_IMAGE_HEIGHT; ?>” alt=”” />

3- Add your Alt Tag to to the portion

alt=”” />

4- It should look like this: (though insert your own site info)

<img src=”<?php header_image(); ?>” width=”<?php echo HEADER_IMAGE_WIDTH; ?>” height=”<?php echo HEADER_IMAGE_HEIGHT; ?>” alt=”Simple Inbound – Marketing | SEO” />